fix(storage): normalize timestamps to UTC to prevent validation failures (#1123)

All time.Now() calls in the dolt storage layer now use time.Now().UTC()
to ensure consistent timezone handling. Previously, timestamps could be
stored with mixed timezone formats (UTC 'Z' vs local '+01:00'), causing
bv validation to fail when updated_at appeared earlier than created_at
in absolute time.

Files modified:
- transaction.go: CreateIssue, UpdateIssue, CloseIssue
- issues.go: CreateIssue, CreateIssues, UpdateIssue, CloseIssue, markDirty, manageClosedAt
- rename.go: UpdateIssueID (2 locations)
- events.go: AddIssueComment (2 locations)
- dirty.go: SetExportHash
- queries.go: Overdue filter, GetStaleIssues

Fixes: bd-84gw9

Co-authored-by: LoomDeBWiles <loomenwiles@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/dirty.go

@@ -82,7 +82,7 @@ func (s *DoltStore) SetExportHash(ctx context.Context, issueID, contentHash stri
 		INSERT INTO export_hashes (issue_id, content_hash, exported_at)
 		VALUES (?, ?, ?)
 		ON DUPLICATE KEY UPDATE content_hash = VALUES(content_hash), exported_at = VALUES(exported_at)
-	`, issueID, contentHash, time.Now())
+	`, issueID, contentHash, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("failed to set export hash: %w", err)
 	}